Oligomeric intermediates of amyloid-? (A?) peptide formed on the pathway of A? aggregation have been suggested to have potent neurotoxic activities in Alzheimer's disease (AD). The hydrophilic and charge-rich N-terminal region of A?, which is highly disordered in amyloid fibrillar structure, has been recently suggested to play an important role along the aggregation pathway. However, the mechanistic details and dynamic regulating function of the N-terminal region in A? aggregation still remain unclear. We hypothesize that the amphiphilic property of the N-terminal sequence is crucial for its regulating function in A? aggregation. Particularly, electrostatic interactions that the N-terminal charged residues are involved in, play a critical role in directing A? early assembly and determining the morphology and metastability of oligomeric intermediates. The objective of this proposal is to clarify the mechanistic details of the N-terminus in A? aggregation including the local aggregation dynamics and conformational transition by using sensitive optical probes, and identify the driving forces that direct A? self-association by employing systematic site-directed mutagenesis in conjunction with biophysical and biochemical approaches. An unnatural amino acid p-cyanophenyl-alanine will be used to elucidate the local environmental changes and conformational dynamics along the aggregation at a residue-specific level. The Specific Aims are: (1) Determine local dynamics and conformational transition of the N-terminal region in A? aggregation; (2) Identify crucial interactions at the N-terminal region in A? oligomerization and fibrillization; and (3) Determine the effect of interplay between A? and charged macromolecules on A? aggregation. The outcome of the proposed research will complement and expand the current efforts toward a comprehensive understanding of the mechanism(s) of A? aggregation and the structure-toxicity relationship for the oligomeric intermediates. In addition, we expect that the knowledge from this research will facilitate the identification of novel targets, such as the key residues and crucial interactions at the N-terminal region, for rational design of strategies to modulate A? aggregation, which may ultimately lead to the development of new therapeutic treatment for AD.